::NTS. 



DHIOOBIT. 



The direction of vibration in jmUriaecl light it not itself 

 cognisable by the aeon*. ID the theory front which Fresnel deduced 

 the mlwve Un, it U suppoaed that the direction of vibration ii perpen- 

 dicular to the plane of poUrisatiun. 



The deduction of the fonn of the wave-surface become! now * mere 

 geometrical problem of envelopes. Freml did not succeed in solving 

 the problem directly on account of the difficulty of the elimination, 

 but he gave a very elegant construction by points of a surface which 

 he afterwards proved to be the wave-surface required, by (bowing that 

 it satisfied the requisite condition as to tangent planes, and he thus 

 obtained iU equation. The construction is as follows. Construct the 

 ellipsoid a~ ! .r* + 6 -1 y* + 0-*** = 1, cut it by a diametral plane, and 

 from the centra perpendicular to this plane draw lines equal respectively 

 to the semi-axes of the elliptic section : the locus of their extremities 

 will be the ware-surface. In the 'Cambridge Philosophical Trans- 

 actions,' vol. vi., p. 85, Mr. A. Smith has very simply obtained the 

 equation of the surface, regarded as an envelope, by direct elimination. 

 The equation U 



oV &V cM 



r_ i + rJ-fcs + rJ_c3 - 



where r f =s** + j* + f. It to readily seen that when two of the para- 

 meters, a, 6, c, become equal, the wave-surface of Kresncl becomes 

 the sphere and spheroid of Huygens. For an admirable dynamical 

 investigation of the problem of double refraction, the reader is referred 

 to a paper by Green, in the 7th vol. of tho ' Cambridge Philosophical 

 Transactions.' 



The length to which this article has already run, compels us to 

 omit the subjects of conical refraction, the application of the undulatory 

 theory to the determination of the intensities of reflected and refracted 

 polarised light, and of the change of phase which accompanies total 

 internal reflection, the properties of metals in relation to the reflection 

 of light, the optical properties of syrup of sugar and other active 

 liquids, and those of transparent uncrystallised media subject to the 

 action of a powerful magnet. For these, reference must be made 

 cither to the original memoirs of those who have investigated these 

 subjects, or to some of the extensive treatises which have been written 

 on the undulatory theory. 



UNWKNTS, or ointments, are unctuous substances, for external 

 Application, and intended to answer a variety of purposes, according to 

 their composition. They are variously designated according to their 

 nature and consistence. At one stage of surgery they were of a very 

 complicated kind, as may still be seen in the so-called balsams (artificial) 

 of the continental pharmacopoeias ; but the progress of modern science 

 leading to greater simplicity, their numbers are much reduced, and 

 their ingredients fewer. If they are composed chiefly of wax, without 

 resin, and of oil, with or without other more active materials, and have 

 a consistence nearly as great as that of plasters, they are termed 

 emttft; when fats or resin are used, so that the consistence is scarcely 

 greater than that of butter, they are termed ointments; if distilled 

 fragrant waters or essential oils are used, they are termed pomatnmt, 

 or pomade*; and occasionally, if the appearance correspond, some are 

 termed butter*, or patten, such as almond patte, which, being bland and 

 emollient, must be carefully ditinguihed from some other compounds 

 also called butters [BuTTKRS, in Pharmacy], which are acrid and corro- 

 sive, such as butter of antimony, or violent poisons, such as butter of 

 arsenic. Most ointments are formed by melting together the ingre- 

 dient-, and in doing this the heat should never exceed that of the 

 boiling-point of water. To ensure uniformity and smoothness, the in- 

 gredients should be carefully stirred while on the fire, and strained 

 through a cloth while yet in the liquid state : if essential oils are used, 

 these must be added afterwards. Some are made by merely triturating 

 the materials together, as in the case of mercurial ointment. No great 

 <|iiantity of any ointment should be prepared at one time, as they are 

 apt to undergo changes, sometimes very detrimental, either by the 

 ingredients acting on each other, or by absorbing oxygen from 

 Many should be prepared only when wanted, such as the ointment of 

 the nitrate of silver. 



The chief use of ointments is either by their emollient qualities to 

 soften tense or hardened part*, or to sheath excoriated parts from acrid 

 secretions or the irritation of tho air. Applied to ulcers, they may, 

 according to their nature, besides excluding the air, promote the heal- 

 ing, if judiciously lined, it hinder it if improperly used. 

 hands or rough skins, one of the mildest and safest applications is 

 almond -paste. 



> . The hoof of a horse looks like the part of a cone'which 

 is separated from the part containing the vertex by an oblique 



b a solid 1s called an unguhi, and rules for the determination 

 nt are given in books of mensuration. 



'] 



I'M Concerning the Unicorn, different opinion* prevail 



among authors," says the author of ' Thaumatographia Naturalis ' 

 (1633;, and he adds that some doubt, others deny, and a third class 

 affirm it* existence. 



Ctcsuui, the author protably whom Aristotle followed, describes the 

 Wild AMMI of India (6m lypwi) as equal to the horse in size, and even 

 larger, with white bodies, red head*, bluish eyes, and a horn on the 

 forehead a cubit in length. For the space of two palms from the fore- 



head this horn U entirely white, the middle part U black, and the 

 extremity is red and pointed. Drinking- vessels are made of i 

 those who use them are subject neither to convulsions, epilep> 

 poison, provided that before taking the poison, or after, they drink 

 from these cups water, wine, or any other liquor. After some 

 particulars, Cteaiai describes these animals as very swift and 

 itrong. (Ctesias, ed. Bohr, pp. 256, 829, 368.) 



Aristotle notices the Indian Ass as a solipede which ha* a horn, and 

 the only one of the solipede* possessing an astragalus. (' Hist A 

 ii. 1.) He adds, in the third book, on the parts of animal*, that those 

 beasts which have only a single horn have it in the middle of 

 head ; and evidently speak* of the Indian Ass from the accounts of 

 others. 



Herodotus (IT. 191) mention* asses (Sroi) having horn* ; and Strabo 

 (xv., p. 1009, Oxford, folio) refer* to Unicorn hone* with the head* of 

 deer. 



Oppian (' Cyneget.,' it, line 96) notice* the Aonian bulls with undi- 

 vided hoofs and a single median hom between their temple*, whereas 

 the Armenian bulls have two. 



Ctesar (' De Hello Oallico,' vi. 26), when referring to the multitude of 

 animals bred in the great Hercynian forest, speaks, probably 

 hearsay, of an ox with the figure of a deer, from the mi. I. He of 

 forehead a single horn stands out higher and more direct tlrm :uiy 

 horns known to him. He adds, that from the top of this horn I 

 like palms are diffused, that the nature of the male 

 same, and that the form and size of their horns are similar. 1 1 

 notices the Elk. 



Pliny, who, to be sure, places it in the company of the ; 

 the Caloblepat, and the IliailM, notices it as a very frr 

 (asperrimam feram), similar in the rest of its body to a horse, with the 

 head of a deer, the feet of an elephant, the tail of a boar, a d< 

 lowing voice (mugitu gravi), and a single black horn, two cm 

 length, standing out in the middle of its forehead. He odd-. 

 feram vivam negant capi," "that it cannot be taken alive" 

 Hist.,' viii. 21) ; and some such excuse may have been necessary in 

 those days for not producing the living animal upon the arena of the 

 amphitheatre. 



Out of this passage most of the modern Unicorns have been described 

 and figured. But let us pause to scan it. The body of the horse and 

 the head of the deer appear to be but vague sketches. The feet of tho 

 elephant and the tail of a boar point at once to a pachydern 

 animal ; and the single black horn, allowing for a little exaggera- 

 te its length, well tits the two last-mentioned conditions, and will 

 apply to one of tho species of Rhinoceros. [RHINOCKUOS, in 

 HIST. Div.] 



Our limits will not permit us to follow out in detail the descri 

 of the numerous writers who have treated of this subject, among 

 are .iKlian, 1'hilostratus, and Solinus, .(Eneas Syh 

 Gesner, Cardan. Garzias ab Horto, Andreas Marinus, Andreas 1! i 

 Bartholinus, Aldrovamlus, Jonston, Ac. Some, however, of the u 

 descriptions of the Unicorn may be cxcepted. Garcia* noted di 

 description of this marvellous creature from one who alleged that lie 

 had seen it. The seer affirmed that it was endowed with a wonderful 

 horn, which it would sometimes turn to the left and right, at 

 raise, and then again depress. Ludovicus Vartomantis write* that he 

 saw two sent to the Sultan from Ethiopia, and kept in a repository at 

 Mahomet's tomb in Mecca, and he describes them as " cancellis 

 tt>s. minime feroces." Cardan describes the Unicorn as a rare animal, 

 of the size of a horse, with hair very like unto that of a weasel, with 

 the head of a deer, on which one horn grows, three cubits in lei, 

 story seldom loses anything in its progress) from the forehead, anq,], 

 at its lowest part and tapering to a point ; with a short neck, a very 

 thin mane, leaning to one cido only, and legs Uiin as those of a . 

 Roe (capreoluM. li.it, not to weary the reader, if he wishes to see 

 what our ancestors thought Unicorns like, let him turn to Jonston's 

 ' Historia Naturalis' (lC. r >7). There he will behold the gmooth : 

 Solipede (" Wald K*el "), and the digitated and cl.-iwcd smooth 1 

 " Meer Wolff," the latter with his single horn erect in the fore;, i 

 but with it depressed in the background, where he ic i 

 regaling on serpents. Then there are the smooth-horned " Monoceros, 

 Unicornu, Kinhorn," with the head, mane, and tail of a hoi 

 lii.iilcato feet; and another smooth-homed " Monoceroa, Un. 

 Kinhorn," with a horse's head and mane, a pig's tail, and camel like 

 feet; the " Mccr Steinbock, Capricornus marimis," with v. 

 cate feet, posterior webbed feet, and a kind of graduated horn 

 modern flat telescope opera-glass pulled out, in the. I'm, 

 charging the ' valiantly in the water in t ! 



i the digitated " \Va|,i r AMmvaudi," with a 



head and two rhinoceros-like horns, one on his forehead ami tl,. 

 on his nose, and a horse's tail, with a eollar round hi- n, , k : I 

 we find the " Moiinceros, scu ricarnu julmtiu- Kinh"in mit ma] 

 with a neek entirely shaggy, a twisted horn, anterior ), 

 tho posterior being webbedTand a deer's tail ; and at the bottom of the 

 plate, "Monoceros, sen Unicornu aliud Kinhorn mit m.ihnen. ein 

 aiidr'art" with a twisted horn, mane, and shaggy gorget, cnrlj t.iil, 

 and camcl-lik, 



Unicom seems to have been a sad puzzle to the hunter 

 hardly know how to come at so valuable a piece of game. It w , 



